1. Field of the Invention
This invention relates to a continuously variable transmission (CVT) in which a slippable start clutch is utilized as an active torsional damping device. More particularly, this invention relates to a simplified adaptive control system that reduces torsional vibrations in the CVT to an acceptable level by measuring torsional vibrations at the clutch output shaft.
2. Description of the Prior Art
A continuously variable transmission system utilizes a pair of adjustable pulleys, including a primary pulley and a secondary pulley. Each pulley has at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material interconnects the pulleys. The inner faces of the sheaves of the pulley are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. The primary pulley is driven by an engine and the output of the secondary pulley drives the drive train of the vehicle. The drive train is typically connected to the secondary pulley through a clutch.
The displaceable sheave of each pulley includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. Increasing the fluid in the chamber increases the effective diameter of the pulley. As fluid is exhausted from the chamber, the pulley diameter decreases. Generally the effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the other.
The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The hydraulic pressure of the sheave of the secondary pulley servo varies the clamping force on the belt connecting the primary and secondary pulleys. U.S. Pat. No. 4,433,594, entitled "Variable Pulley Transmission", provides further information in relation to CVTs and is incorporated herein by reference in terms of background to the present invention.
Engines that drive continuously variable transmissions, especially reciprocating engines, tend to produce torque fluctuations that lead to torsional vibrations on the input shaft of the belt and sheave system. It is important to the smooth operation of the continuously variable transmission that such torsional vibrations are kept within acceptable limits.
The manner in which vibrations are reduced or dampened is dependent, in part, on the location of the different elements of the CVT. In a system such as that described in U.S. Pat. No. 4,433,594, which includes a slippable start clutch downstream of the belt and sheave system, a long travel damper is generally located up-stream of the belt and sheave system to damp start-up, shut-down and drive mode vibrations, thereby preventing transmission of such vibrations to the belt and sheave system. In a continuously variable transmission having a start clutch positioned up-stream of the belt and sheave system, the larger torsional vibrations experienced during start-up and shut-down are not transmitted as a result of the slip between the clutch plates. This is to some extent a by-product of clutch control primarily aimed at ensuring that torque transmitted to the belt and sheave system increases or decreases smoothly during start-up and shut-down, respectively. Once the transmitted torque has attained the required level, the clutch is locked. A short travel mechanical damper is generally located upstream of the belt and sheave system to damp the vibrations experienced during clutch lock-up.
U.S. Pat. No. 4,665,773, entitled "Continuously Variable Transmission Apparatus for Automobile", describes an alternative configuration of a CVT which includes a wet start clutch positioned up-stream of the belt and sheave system. A control system regulates the hydraulic circuit that controls slip across the start clutch. During the drive mode, the clutch is substantially locked to allow selected slip in accordance with signals from the control unit. Empirically collected data is used to provide information on the level of vibrations likely to be experienced by the continuously variable transmission for a range of engine conditions defined in terms of a set of engine parameters. These parameters include the degree of opening of the throttle valve, the engine torque and the engine speed. For a given set of measured engine parameters the probable level of vibration experienced by the continuously variable transmission is determined and a requisite amount of slip to remove that level of vibration is calculated. The clutch slippage is regulated by operation of a hydraulic fluid circuit.
The present invention avoids the need for empirical data by providing an adaptive system to control the slip in the start clutch.